The present invention relates to machine tools and, more particularly, to machine tools having a motor driven tool for engaging a work piece and an electromagnetic base that can be selectively energized to magnetically engage a work surface.
Magnetic base machine tools typically include a electromagnetic support base that can be energized to engage a work surface and a drive motor assembly mounted on the base. A tool bit, such as a drill bit, is coupled to the drive motor assembly and is used to machine the work surface or a workpiece after the magnetic base has been energized. The magnetic base typically includes a flux producing magnet coil and a flux concentrating core. When an electric current of suitable magnitude is passed through the magnet coil, a magnetic flux is produced to firmly attach the magnetic base to a work surface. Where the machine tool is a drilling machine, the drive motor assembly is typically carried on guides for movement along a tool axis with a rack and pinion assembly allowing a user to control the movement of the drive motor and attached tool bit. As the tool bit engages the work surface, it exerts a reaction torque on the energized magnetic base which can cause the magnetic base to slide on the work surface. The reaction torque can be substantial and can arise quite suddenly, for example, when a tool bit stalls in the work surface. In order to counteract the reaction torque, it is common in these type of machines to provide a pointed stabilizer pin that is driven into the work surface to provide a mechanical engagement between the magnetic base and the work surface, the mechanical engagement typically sufficient to counteract any reaction torque produced by the tool bit while it engages the work surface.
Magnetic base machine tools have found utility in fabricating and repairing large steel structures, such as bridgeworks and ships, where the magnetic base can conveniently secure the machine tool to horizontal, vertical, and even overhead surfaces to allow machining that might otherwise be difficult or impossible to accomplish using hand-held power tools.
There are a number of criteria applicable when designing a magnetic base machine tool, such as a drilling or boring machine. To prevent the support base from sliding on the work surface in response to the reaction torque developed by the tool bit, it is important that the magnet coil in the base be energized and the stabilizer pin be in place to secure the tool to the work surface prior to operation of the tool driving motor. In addition, the magnetic base should be kept energized during run down of the tool motor to prevent breakaway if the magnet coil is de-energized before the tool motor comes to a halt. In those instances where the magnetic base is energized for a considerable period of time, a permanent magnetic set can be induced in the work surface. In this instance, it is desirable to momentarily reverse the magnetic field applied by the magnetic base to facilitate removal of the base from the work surface.
In the past, various controls and switching devices have been provided to effect the various sequential control operations. The use of plural, separately operated controls can be susceptible to mis-operation, particularly by untrained operators. In order to optimize operation, it is desirable that any control unit cause all the functional steps that must be taken to occur in the proper sequence and in such a way that out-of-sequence operation or mis-operation is impossible. It is also preferable, from the standpoint of user convenience, that the control unit be operable with one-hand only to enhance the ease of operation.